Fortinet FortiGate FortiGate-800 Installation And Configuration Manual page 57

NAT/Route mode installation
FortiGate-800 Installation and Configuration Guide
Restricting access to a single Internet connection
In some cases you might want to limit some traffic to being able to use only one
Internet connection. For example, in the topology shown in
organization might want its mail server to be able to connect to only the SMTP mail
server of ISP1. To do this, you add a single Internal->External firewall policy for
SMTP connections. Because redundant policies have not been added, SMTP traffic
from the Internet network is always connected to ISP1. If the connection to ISP1 fails
the SMTP connection is not available.
Configuration example: Multiple connections to the Internet
Figure 9 on page 51 the
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